Waveguide stop-band filter utilizing hybrid circuit with lossy resonant cavities in branch arms



United States Patent 3,142,028 WAVEGUIDE STOP-BAND FILTER UTILIZINGHYBRID CIRCUIT WITH LOSSY RESONANT CAVITIES IN BRANCH ARMS Robert D.Wanselow, Woodiand Hills, Califi, assignor to Hughes Aircraft Company,Culver City, Calif., a corporation of Delaware Filed May 31, 1962, Ser.No. 198,870

5 Claims. (Cl. 333-73) The present invention relates to waveguidecomponents, and more particularly relates to a hybrid stop-band filterfor introducing frequency sensitive attenuation into a waveguide.

In the prior art, frequency sensitive attenuation of microwave energyhas been achieved by directional filter devices which attenuate apreselected band of microwave frequencies with minimum reflection attheir input port. However, in such devices the reflection coefficientvaries inversely with the presecribed attenuation, and in addition,passband attenuation is difiicult to control.

Accordingly, it is an object of the present invention to provide a novelwaveguide stop-band filter which provides attenuation over a readilycontrollable range of frequencies.

It is a further object of the present invention to provide a waveguidestop-band filter for attenuating a specified amount of microwave energywithin a predetermined frequency range with a minimum amount ofreflection of microwave energy in the stop-band range, and in which thereflection coefiicient is independent of the amount of prescribedattenuation.

It is a still further object of the present invention to provide adevice for introducing into a waveguide frequency sensitive loss whichis smaller and lighter than prior art devices and capable of providingcomparable attenuation over a comparable frequency range.

The objectives set forth above are achieved by the present invention byutilizing an attenuation-containing cavity resonator device incombination with a waveguide hybrid to provide frequency sensitive lossfor electromagnetic waves propagating through the hybrid. The hybridin-' cludes an input port, an output port, a first auxiliary port and asecond auxiliary port, with electromagnetic waves of frequencies withina predetermined frequency range being propagated through the hybridbetween the input port and the output port. A first cavity which is maderesonant at a preselected frequency at which attenuation is to beintroduced is coupled to the first auxiliary port, and an identicalresonant cavity is coupled to the second auxiliary port of the hybrid.Lossy dielectric material is disposed in each resonant cavity toattenuate electromagnetic energy over a preselected frequency range.

Other and further objects, advantages, and characteristic features ofthe present invention will become readily apparent from the followingdetailed description of a preferred embodiment of the invention whentaken in conjunction with the appended drawing in which:

FIG. 1 is a plan view in longitudinal section illustrating a waveguidestop-band filter constructed in accordance with the principles of thepresent invention; and

FIG. 2 is a longitudinal sectional view taken along 22 of FIG. 1.

Referring now to FIGS. 1 and 2 with more particularity, the waveguidestop-band filter of the present invention comprises a four-terminalhybrid network designated generally by the reference numeral and acavity resonator circuit 12 electromagnetically coupled to two of theterminals of the hybrid 10. The hybrid 10 is preferably a conventional 3db short slot forward wave directional coupler having an input port 14,an output port 16, and intermediate, or auxiliary, ports 18 and 20.Intermediate port 18 is aligned with input port 14 on one side of thehybrid 10, while intermediate port 20 is similarly aligned with outputport 16 on the other side of the hybrid 10. The hybrid 10 is of lessoverall width in its central region than in its end portions adjacentthe ports 14-16 and 18-20, respectively. Electrically conductive plates24 and 26 project into the hybrid 10 from opposite ends thereof in aplane parallel to the length of the hybrid to define a first Waveguidingpassage 17 between ports 14 and 18, a second waveguiding passage 19between ports 20 and 16, and a coupling aperture, slot, or iris, 28between the regions 17 and 19.

A rectangular input waveguide 30 is disposed adjacent the input port 14of the hybrid 10 to carry input microwave energy to the hybrid, and arectangular output waveguide 32 is similarly disposed adjacent theoutput port 16 of the hybrid 10 to propagate output microwave energyfrom the hybrid. Inner walls 34 and 36 of the input and outputwaveguides 30 and 32, respectively, are contiguous to one another andlie in essentially the same plane as that which contains plates 24 and26.

Frequency sensitive attenuation is' introduced into the filterarrangement by means of the cavity resonator circuit 12. Morespecifically, the cavity resonator arrangement 12 comprises a block 15of electrically conductive material which defines a pair of cylindricalcavities 38 and 40. Cavities 38 and 40 are coupled to the intermediateports 18 and 20, respectively, of the hybrid 10 by means of respectiverectangular waveguides 42 and 44. The waveguide 42 is aligned with andhas the same cross-section as input waveguide 30, while the waveguide 44is aligned with and is of the same crosssection as output waveguide 32.Inner walls 46 and 48 of waveguides 42 and 44, respectively, aredisposed contiguous to one another in the plane of plates 24 and 26. Thecylindrical cavities 38 and 40 are disposed with their respectivelongitudinal axes parallel to a line defined by the intersection of theplane containing plates 24 and 26 with the plane containing ports 18-and 20. Cylindrical cavity 38 is inductively coupled to waveguide 42 bymeans of coupling iris 50 in conductive block 15, and similarly,coupling iris 52 in block 15 inductively connects the cylindrical cavity40 with the waveguide 44.

In order to provide the desired attenuation, cylindrical attenuatorblocks 54 and 56 of lossy dielectric material are disposed in thecavities 38 and 40, respectively. The dimensions of the blocks 54 and 56are essentially the same as those of the cavities 38 and 40 so that thecavities 38 and 40 are essentially completely filled with lossydielectric material. An example of a material which may be used for theblocks 54 and 56 is a mixture of forsterite and silicon carbide, withthe percentage of silicon carbide varying from essentially 3% toessentially 10%. Examples of other materials which could be used aresilicon carbide and alumina, silicon carbide and talc, or otherdielectric and lossy material combinations.

The cavities 38 and 40 are each of a radius R and are designed toresonate in the TM cylindrical cavity mode, with the radius R beingdetermined by the desired resonant frequency. More specifically, for agiven resonant frequency i (in kmc.) the cavity radius R (in inches) isgiven by the equation R=4.52l/(f where e is the relative dielectricconstant of the lossy dielectric material filling the resonant cavity.

In the operation of the waveguide filter of the present invention,microwave energy of frequencies within the relatively broad frequencypassband of the hybrid 10 will propagate through the hybrid in thefollowing manner. Energy entering the input port 14 from waveguide 30divides into two portions, with essentially half of the input energytraveling straight through region 17 of the hybrid to intermediate port18, and the other half propagating through coupling iris 28 and region19 to the intermediate port 20. The longer propagation path betweenports 14 and 20 introduces a phase shift such that microwave energyarriving at intermediate port 20 lags the energy arriving at port 18 by90.

Microwaves of frequencies other than the cavity resonant frequency fwhich pass through ports 18 and 20 travel along the respectivewaveguides 42 and 44, and are reflected by the walls of conductive block15 back toward the intermediate ports 18 and 20, with the reflectedenergy arriving at port 20 lagging the reflected energy arriving at port18 by 90. This reflected energy enters the hybrid through ports 18 and21) and traverses the length of the hybrid it) in the opposite directionthan before. Microwave energy from each of the auxiliary ports 18 and 20divides essentially in half, with half propagating directly through thehybrid 10 to the aligned port at the opposite end of the hybrid and theother half crossing to the other side of hybrid via the iris 28. Since a90 phase lag is experienced each time energy traverses the iris 28,energy arriving at input port 14 from auxiliary port 20 will be 180 outof phase with energy arriving there from auxiliary port 18. Hence,destructive interference will occur, and no energy will be directed outof port 14 into the input waveguide 30. On the other hand, energy fromports 20 and 18 will arrive at the output port 16 in phase, and theresultant sum of this energy will pass through the port 16 into theoutput waveguide 32.

Input energy in the vicinity of the resonant frequency i will travelthrough the hybrid 10 from input port 14 to the auxiliary ports 18 and20 in the manner described above. However, after traversing thewaveguides 42 and 44, this energy will not be reflected by the walls ofthe conductive block back toward the hybrid 10 but will be directedthrough the respective irises 50 and 52 into the resonant cavities 38and 40 where it will be substantially absorbed by the respective lossyattenuators 54 and 56. Hence, a minimum amount of energy in the vicinityof f will be propagated back to the hybrid 10 and will reach the outputport 16.

Thus, the filter of the present invention serves to essentially passmicrowave energy of frequencies within a relatively broad band offrequencies while attenuating such energy in the vicinity of apreselected frequency, thereby introducing a stop-band about theselected frequency. The amount of loss introduced at the resonantfrequency f can be readily controlled by varying the composition of theattenuators 54 and 56 or by changing the geometry of the coupling irises50 and 52 between waveguides 42 and 44 and the resonant cavities 38 and40, respectively. Moreover, as the composition of the attenuatingmaterial is changed the Q of the resonant circuit will be affected,i.e., as the percentage of lossy material relative to dielectricmaterial is increased the Q will decrease. Thus, altering the attenuatorcomposition may be used to adjust the width of the stop-band.

Although the present invention has been shown and described withreference to a particular embodiment, it is to be understood thatchanges or alterations obvious to one skilled in the art to which thisinvention pertains are, nevertheless, within the spirit and scope of theinvention as set forth in the appended claims.

What is claimed is:

1. A waveguide filter for providing a stop-band of preselected width forelectromagnetic energy in the vicinity of a preselected frequencycomprising in combination: a waveguide hybrid device for propagatingelectromagnetic energy between an input port and an output port, meansdefining at least one cavity coupled to said hybrid device, said cavitybeing resonant at said preseletced frequency, and loss means disposed insaid cavity for attenuating electromagnetic energy in the vicinity ofsaid preselected frequency, said loss means comprising a mixture ofdielectric and lossy materials in a preselected proportion to provide apreselected cavity Q and a corresponding preselected stop-band width.

2. The combination according to claim 1 wherein said cavity is of acylindrical shape and is resonant in the TM mode.

3. The combination according to claim 1 wherein said loss means is ablock having dimensions substantially the same as the dimensions of saidcavity and comprising a mixture of essentially between and 97%forsterite and essentially between 3% and 10% silicon carbide.

4. A waveguide filter for providing a stop-band of preselected width forelectromagnetic energy in the vicinity of a preselected frequencycomprising in com bination: a waveguide hybrid device having an inputport, an output port, a third port disposed opposite said input port anda fourth port disposed opposite said output port, for propagatingelectromagnetic energy between said input port and said output port,means defining a first cavity coupled to said third port and a secondcavity coupled to said fourth port, said first and said second cavitiesbeing resonant at said preselected frequency, and loss means disposed insaid first and said second cavities for attenuating electromagneticenergy in the vicinity of said preselected frequency, said loss meanscomprising a mixture of dielectric and lossy materials in a preselectedproportion to provide a preselected cavity Q and a correspondingpreselected stopband width.

5. A waveguide component for providing a stop-band of preselected widthfor microwave energy in the vicinity of a preselected frequencycomprising: a coupling circuit having an input port, an output port, afirst auxiliary port, and a second auxiliary port; said input port andsaid output port being symmetrically disposed at one end of saidcoupling circuit and said first and second aux iliary ports beingsymmetrically disposed at the opposite end of said coupling circuit;said coupling circuit further defining a first waveguiding passage ofsubstantially rectangular cross-section between said input port and saidfirst auxiliary port, a second waveguiding passage of substantiallyrectangular cross-section between said second auxiliary port said outputport, and a coupling aperture between said first and second passages,whereby microwave energy of frequencies within a predetermined frequencyrange is propagated between said input port and said output port; afirst waveguide of rectangular cross-section electromagnetically coupledto said first auxiliary port; a second waveguide of rectangularcrosssection electromagnetically coupled to said second auxiliary port;a conductive member disposed at the ends of said first and secondwaveguides remote from said first and second auxiliary ports; saidconductive member defining a first cylindrical cavity and a firstcoupling aperture electromagnetically coupling said first cylindricalcavity to said first waveguide; said conductive member further defininga second cylindrical cavity and a second coupling apertureelectromagnetically coupling said second cylindrical cavity to saidsecond waveguide; each said cylindrical cavity being resonant at saidpreselected frequency; and a cylindrical block of a mixture ofdielectric and lossy materials in a preselected proportion to provide apreselected cavity Q and a corresponding preselected stop-band widthsubstantially filling each said resonant cavity.

References Cited in the file of this patent UNITED STATES PATENTS2,293,839 Linder Aug. 25, 1942 2,916,712 Artuso Dec. 8, 1959 2,951,216Nelson et al. Aug. 30, 1960 3,034,076 Tomiyasu May 8, 1962 3,041,542Bailey June 26, 1962

1. A WAVEGUIDE FILTER FOR PROVIDING A STOP-BAND OF PRESELECTED WIDTH FORELECTROMAGNETIC ENERGY IN THE VICINITY OF A PRESELECTED FREQUENCYCOMPRISING IN COMBINATION: A WAVEGUIDE HYBRID DEVICE FOR PROPAGATINGELECTROMAGNETIC ENERGY BETWEEN AN INPUT PORT AND AN OUTPUT PORT, MEANSDEFINING AT LEAST ONE CAVITY COUPLED TO SAID HYBRID DEVICE, SAID CAVITYBEING RESONANT AT SAID PRESELECTED FREQUENCY, AND LOSS MEANS DISPOSED INSAID CAVITY FOR ATTENUATING ELECTROMAGNETIC ENERGY IN THE VICINITY OFSAID PRESELECTED FREQUENCY, SAID LOSS MEANS COMPRISING A MIXTURE OFDIELECTRIC AND LOSSY MATERIALS IN A PRESELECTED PROPORTION TO PROVIDE APRESELECTED CAVITY Q AND A CORRESPONDING PRESELECTED STOP-BAND WIDTH.